Deuteronomy 16 covers the three yearly feast times which were required attendance for all Jewish males of fighting age. The Feast of Passover and Unleavened Bread was specifically a memorial of the deliverance from Egypt. The day after the Sabbath after Passover was the Feast of Firstfruits, from which the 50 days were counted to the Feast of Weeks, or Pentecost. Pentecost celebrated the reaping of the harvest from the land that God had given them and also served as a reminder that they had been slaves in a foreign land. The Feast of Booths recreated the camp of Israel around the tabernacle of God as they had been in the wilderness after the exodus.
Paul says in Colossians 2 that this was all elementary school. These feasts and regulations were given as instruction, but were only shadows of things to come. The substance belongs to Christ. He is the fulfillment of all the Old Testament, just as He says, “I have not come to abolish the Law or the Prophets but to fulfill them,” and, “You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me.”
“Christ, our Passover lamb, has been sacrificed.” He is our replacement firstborn, killed for us, and His blood opens the door for us and delivers us from bondage to sin and the fear of death. He is the Firstfruits, the firstborn from the dead, the first grain of the harvest given to God so that the rest might be brought in. He tells His disciples to wait in Jerusalem and He pours out His Spirit on them on Pentecost so that they might become the bounty and fullness of the land. And He is Immanuel; He came to pitch His tent with us so that we might join with the great cloud of witnesses that surrounds Him, the army of saints encamped around our King.